Compact integrated sound systems, such as those incorporated in personal computers, portable televisions and portable stereos, generally provide limited physical separation between the stereo speakers and a correspondingly small angle between the speakers and the listener. It follows that with such compact sound systems the "spatialization", which is defined as the width of the perceived sound stage, will be generally perceived by the listener as inferior to that of larger systems. Additionally, small sound systems generally utilize small stereo loudspeakers with limited low-frequency capability.
Correlated information is typically generated when a recording engineer balances a lead instrument or instruments equally on both stereo channels so that the listener will perceive such instruments as a center image when reproduced by the stereo speakers.
In small sound systems having limited physical separation between the stereo speakers, the stereo signals can be processed in a manner to enhance spatialization; in such an enhancement signal process, a high degree of rejection of correlated information serves to prevent excessive spatialization of such correlated information, which in turn preserves the center image of lead instruments as intended during the recording process. At the same time, however, a high degree of rejection of correlated information excessively reduces the spatialization of correlated reverberation sound components.
It has been demonstrated that human hearing is most sensitive to spatial information at lower-midrange frequencies and is most sensitive to reverberant information at upper-midrange frequencies. It follows that a high degree of rejection, of correlated information below a lower-midrange frequency and a reduced degree of such rejection of correlated information above such lower midrange frequencies serve to optimally spatialize stereophonic information as well as correlated reverberant information in small sound systems.
It is known that the ability of a loudspeaker to accurately reproduce lower-midrange frequencies is crucial to the reproduction of spatially enhanced music. It is also known that the ability of small loudspeakers to accurately reproduce such lower-midrange frequencies is compromised when low-bass signals are present in the loudspeaker driving signals. Since small loudspeakers are not typically capable of providing audible low-bass acoustic output, such low-bass signals may be attenuated in the left and right loudspeaker driving signals as a means to improve spatial reproduction without affecting the audible bandwidth of the loudspeaker. It has been determined by the present inventor that an optimum low-bass cutoff frequency, typically between 35 Hz and 70 Hz, exists for the left and right stereo signal components of loudspeaker driving signals, and an optimum upper-bass cutoff frequency, typically between 80 160 Hz, exists for the derived difference signal components of such driving signals. This difference in optimum cutoff frequency occurs as a result of the oppositely polarized relationship of the derived left and right difference signals, whereby high-pass filtering at an upper bass frequency is required to prevent stereophonic acoustic cancellation, and a corresponding reduction in sound pressure level below such upper bass frequency.